The accident at Three Mile Island nuclear generating station Unit-2 (TMI-2) has shown that it is important for all light water nuclear power reactors to have some means for determining the density and level of the primary coolant in the pressure vessel. If data from such a system had been available to the TMI-2 operators, the extensive damage which occurred to this unit might have been avoided.
The system should be able to operate under a wide range of temperature, pressure and radiation levels. It should be capable of operating with the reactor at power and in its shutdown mode and should be able to detect changes in coolant density when the pump(s) is/are running and changes in coolant level when the pumps are turned off. To allow for safe yet rapid backfitting into existing reactors, the system should not require any major modifications to the plant such as penetration of the primary system boundary. Various types of level systems are under consideration. Systems have been proposed which involve the use of such concepts as elastic deformation of the pressure vessel, Cerenkov radiation monitoring, and the use of ultrasonic or microwave emission and detection. It is known there are many potentially interfering signals which effect the accuracy and reliability of some of the proposed concepts. The interfering signals originate from such sources as the intense gamma and neutron fields, system vibrations, and the lack of a clear sharp boundary between the steam and water in a light water reactor undergoing a loss-of-coolant accident. Some of the proposed concepts would require new penetrations of the reactor pressure vessel, and most would require extensive and expensive retrofitting. In addition, they would require substantial testing to verify that a chosen solution would provide the desired information under accident conditions and that it would not compromise the safety and performance of the plant.